1. Field of the Invention
This invention relates to latch mechanisms for doors and in particular to door latch mechanisms comprising a latch retraction bar.
2. Description of the Related Art
Door locking mechanisms and security doors to prevent theft or vandalism have evolved over the years from simple doors with heavy duty locks to more sophisticated egress and access control devices. Hardware and systems for limiting and controlling egress and access through doors are generally utilized for theft-prevention or to establish a secured area into which (or from which) entry is limited. For example, retail stores use such secured doors in certain departments (such as, for example, the automotive department) which may not always be manned to prevent thieves from escaping through the door with valuable merchandise. In addition, industrial companies also use such secured exit doors to prevent pilferage of valuable equipment and merchandise.
One type of a commonly used exit device is a push bar or push rail (“push bar”) actuated latch retraction device installed on the inside of a door. When sufficient pressure is applied to the bar it depresses causing the door latch to retract from the door frame, allowing the door to be opened. These types of exit devices are typically required by fire or building codes and are used in public buildings where many people may be gathered. The devices allow for safe and quick egress from inside of the building, such as in the case of an emergency. These devices allow for this egress while keeping the door locked to those trying to enter the building from the outside.
U.S. Pat. No. 6,116,661 describes an electric dogging mechanism for a push bar exit device consisting of slidable plate and armature which are attracted to an electric coil when the coil is energized. The slidable plate is connected to a push bar mechanism. After the push bar is depressed, retracting the exit device latch, the coil is energized attracting and holding the armature to the coil. This holds the push bar depressed and the latch retracted by the connection of the slidable plate to the push bar mechanism.
Electrically operated push bar exit devices can also be used in applications where they can be operated by a card reader or keypad from outside to allow access through a door that also serves as an exit bar latch retraction device from inside. Other applications allow for these devices to allow operation with power door operators, allowing the latch to retract on command such as through a timed schedule. These timed schedules can be implemented at facilities that operate on a fixed schedule, such as schools.
Some current implementations of these electric exit devices utilize solenoids to retract the latch bolt, which can require a relatively high operating current to reliably retract the latch bolt and overcome initial friction. Another current implementation uses a motor to retract the latch bolt, with the motor pulling back the bar which causes the latch bolt to retract. A switch can be included to detect when the bar reaches the fully retracted position, at which point the motor is turned off. In this design, the motor does not shut off until the push rail is fully retracted as sensed by the switch. Internal components of the exit device can bind or otherwise prevent the motor from fully retracting the latch bolt. This causes the motor to overwork and produces a continuous drive to the motor which can ultimately burn it out.
PCT International Publication No. WO 2008/010876 A2 to Sargent teaches that a stepper motor type linear actuator can be used and the retraction distance can electronically be controlled by counting the rotational steps of the motor. This electronic monitoring or “rotation counting” is claimed to be superior to having a fixed switch controlling the motor function however it is simply a means of eliminating the separate monitor switch and still suffers the same susceptibility to failure from wear of mechanical parts through the life of the device. The electronic user adjustable latch control can create more of an opportunity for device problems through miss adjustment. The means of holding the latch and linear actuator in the retracted position is accomplished by electronically holding the stepper motor in the actuator in a “stalled mode”. This requires that the motor remains powered.
PCT International Publication WO 2006/015769 to Dorma also discloses a means of retracting the latch through linear movement. There is no mention of how the patented mechanism would be held in the retracted position.